Once again Mark Twain's Library a reconstruction by Alan Gribben comes in
very handy.  Gribben inspected Twain's copy of Holcombe's History of Marion
County (its in the Berkeley collection), and found one pencil mark on page
902 (Twain didn't sign this book).

Gribben states, "Holcombe wrote to Clemens on 29 August 1883 from Palmyra,
Missouri, enclosing a prospectus for the volume and asking Clemens'
assistance in preparing a sketch of John Marshall Clemens (MTP).  Clemens
sent the letter to Orion Clemens, who contributed to the book."